Plastic optical fibres have attracted much interest and been an area of active commercial research for many years since they offer the potential of combining the rapid data transmission advantages afforded by glass optical fibres with the ruggedness and low costs associated with copper cabling products.
Despite this activity and interest, plastic optical fibres (POFs) have not, to date, been widely accepted as a data communications medium. One of the key contributing factors in delaying this acceptance has been the poor thermal stability of available fibres. Data transmission systems in both automotive and aerospace applications provide large markets for POFs. However, underbonnet applications in automobiles require extended performance above 100.degree. C., with many aerospace environments even more severe and demanding performance at temperatures in excess of 140.degree. C. The upper use temperature for current commercial POFs, which are predominantly based on polymethyl methacrylate [PMMA] and polystyrene [PS], is about 80.degree. C., which is too low for these applications. The high temperature performance of a polymer for these applications is limited by its glass transition temperature (Tg), since at temperatures around and above Tg its mechanical and optical properties decline. For both PMMA and PS, Tg is in the range of 100.degree. C. to 105.degree. C. Thus, materials with Tgs above those of PMMA and PS are desirable to increase the maximum use temperature of the resulting POFs.
A further factor delaying the acceptance of POFs has been the high optical attenuations of currently available commercial fibres to transmission in the red and near infrared (NIR) region, where preferred solid state light sources operate. The fabrication of plastic optical fibres with lower attenuations in these regions is an area of current polymer fibre research.
The predominant contributor to the red and NIR optical attenuation of most amorphous polymeric materials suitable for optical fibres is absorption caused by overtone and combination bands of the C--H bond's fundamental vibration. The academic and patent art has therefore concentrated on reducing these attenuations and this has been achieved by partial or complete replacement of H atoms by the heavier deuterium or halogen atoms. This reduces NIR attenuations due to the higher reduced mass of the C--X bond (X.dbd.D,F,Cl) compared to the C--H bond, so shifting the fundamental and overtone frequencies out of the range of interest.
The replacement of H with D has received much attention and has produced very good results (Appl.Phys.Let. 1983, 42, 567). Attenuations as low as 20 dB/Km (650 nm) have been obtained from fully deuterated PMMA fibres. Unfortunately, cost considerations of deuteration make this approach essentially of academic interest only, and there has been no attempt to commercialise a fibre containing deuterium.
Halogenation represents a much more commercially attractive option to reducing attenuations. By introduction of these heavier atoms, the effect of hydrogen overtones can be reduced and/or diluted and so attenuations may be reduced in a similar manner to deuteration. Whilst both chlorine and bromine substituted polymers have been considered, the advantages associated with fluorination, namely, C--F bond stability, low atomic bulk etc., have made this approach the most attractive.
For a number of reasons, mainly related to fabrication and processing, optical polymers are predominantly acrylic ester based materials. Fluorination in such systems has been largely achieved by the use of short-chain perfluorinated ester methacrylates, taking advantage of the readily available short chain perfluoroalcohols. A review of this art is disclosed in WO 93/03074. The attenuation improvements available from such materials, particularly in the NIR can be significant. The above document discloses materials with NIR attenuations approximately 25% of those of conventional PMMA fibres. However, in order to reduce attenuations substantially further, not only must the side chain ester functionality of the polymer be fluorinated, but it is also necessary to replace the hydrogen atoms of the polymer backbone. One of the most effective methods of achieving this is to replace the methyl function of the methacrylate backbone with a fluorine atom, producing 2-fluoroacrylate polymers. As well as reducing the polymer H-atom content, this approach also has the advantage that, by careful choice of the side-chain group, the Tg of the polymer may be increased to a level where it may be suitable for high temperature usage. The combination of high Tg, leading to a high upper use temperature, and a high degree of fluorination, giving the potential for very low optical losses, yields materials which show much promise as optical fibre core materials for local area networks where the use conditions are severe, for example in automobile and aerospace applications.
Esters of 2-fluoroacrylic acid are well known. The methyl ester of 2-fluoroacrylic acid can be prepared by the reaction of methyl 2-fluoroacetate with formaldehyde in the presence of calcium hydride and dimethyl oxalate, as disclosed in Macromolecules 1980, 13, 1031-1036. The polymerisation of this monomer, methyl 2-fluoroacrylate (MFA), is reported to give a high Tg (128.degree. C.) polymer.
Several methods are known for the preparation of other derivatives of 2-fluoroacrylic acid e.g. J. Fluorine Chem. 1991, 55, 149-162. For example, 2H-octafluorocyclopentyl methyl 2-fluoroacrylate may be prepared from 2,3-difluoropropionyl chloride by a two-step reaction in which the acid chloride is dehydrofluorinated to give the 2-fluoroacryloyl chloride, then treated with 1H,1H,2H-octafluoropentyl methanol in the presence of base. There is no disclosure of polymerisation of this or other monomers although reference is made to the good general optical and physical properties of 2-fluoroacrylic ester polymers.
It is known that polymers containing 2-fluoroacrylate esters may be used for the preparation of optical fibre cores. EP-0128516 discloses monomers of the formula H.sub.2 C.dbd.CF--COOR.sub.f, in which R.sub.f is a fluorine-containing aliphatic group, preferably a fluorine-containing lower alkyl group. Examples are given of 2-fluoroacrylate homopolymers with glass transition temperatures up to 125.degree. C. [poly (3H-1,1-dimethyltetrafluoropropyl 2-fluoroacrylate)]. JP 02092908 discloses 2-fluoroacrylates in which the backbone C--H bonds are replaced by C--D bonds to reduce the NIR bond absorptions. The monomers have the formula D.sub.2 C.dbd.CR.sup.1 --COOR.sup.2, in which R.sup.1 is F, D, or CD.sub.3, and R.sup.2 is C.sub.n Y.sub.2n+1 (Y.dbd.F,Cl) Lower alkyl chains only are exemplified.
Monomers and polymers of 2-fluoroacrylates with fluorinated rings are disclosed in FR 2623510. However, the examples and claims relate only to fluorinated aromatic materials.
AU-A-13035/88 discloses 2-fluoroacrylate materials in which a substituted bicyclic ring is present. The monomer structure allows the possibility of bicyclic perfluorinated 2-fluoroacrylate esters, however, all examples relate to chlorinated monomers. The patent recommends specifically chlorine, bromine or trifluoromethyl substitution, but discloses no perfluorinated rings.
Other examples of polymers of alicyclic highly fluorinated (meth)acrylic monomers are known as optical materials. WO93/03074 discloses a specific monomer, 1H, 1H-perfluorocyclohexylmethyl methacrylate, and homopolymers and copolymers thereof with other fluorinated and non-fluorinated monomers, as an optical fibre core with low optical loss. However, high temperature usage is not disclosed.
U.S. Pat. No. 5,148,511 discloses cladding compositions comprising copolymers of fluorine containing methacrylate monomers with methyl methacrylate. Glass transition temperatures of up to 108.degree. C. are indicated for solution copolymers of 1H,1H-perfluorocyclohexylmethyl methacrylate with methyl methacrylate in 50:50 weight ratio.
The present invention provides alternative fluorine-containing materials suitable for the preparation of optical elements.